The generation of a high affinity protective antibody response requires T and B cell collaboration in specialized areas of secondary lymphoid tissue known as germinal centers. Understanding the regulation of these cellular interactions is important for vaccine development and for understanding the role of germinal center lymphocytes in tumor development and autoimmunity. T follicular helper cells (TFH) are specialized CD4 T cells that are able to migrate into follicles and deliver helper signals to B cells, and thus play a critical role in functional maturation of a humoral immune response. Global analysis of gene expression has shown that both germinal center B cells and TFH have unique patterns of gene expression that distinguish them from other subsets of naive and activated lymphocytes. One of the genes reported to be expressed specifically in both germinal center B cells and TFH encodes TOX, a protein highly conserved between rodents and humans and first identified in our laboratory as a regulator of T cell development. We have produced TOX transgenic mice and globally and conditionally TOX-deficient mice and have shown that TOX is required for development of the CD4 T cell lineage. Using combinations of these genetically modified animals, we propose here to determine whether TOX also plays a role in germinal center reactions. Our approaches allow us to distinguish the effects of TOX-deficiency on B and T cells. Thus, breeding conditionally TOX-deficient mice to mice that express Cre recombinase in the B cell lineage will be used to look for defects in germinal center B cell formation and function upon immunization. Two complementary approaches are proposed to study similarly the role of TOX in TFH development and function;complementation of globally TOX-deficient mice with a thymically expressed TOX transgene and in vitro Cre-mediated deletion of the Tox locus in CD4+ T cells that are subsequently tested for TFH development and function in vivo. To complement these studies we also propose to generate a TOX reporter knock-in strain of mice that will allow us to track expression of TOX in viable cells and in vivo during an immune response. PUBLIC HEALTH RELEVANCE: A protective antibody-mediated immune response requires T and B lymphocyte interactions in specialized areas of secondary lymphoid tissue known as germinal centers. Understanding the regulation of these interactions is important for vaccine development and for understanding the role of germinal center lymphocytes in tumor development and autoimmunity. This research program is directed at understanding the role of a specific nuclear protein, highly conserved in rodents and humans, in regulating these germinal center reactions.